1. Field of the Invention
This invention relates to electrical inductive apparatus, and more particularly it pertains to insulated pancake coils.
2. Description of the Prior Art
Those skilled in transformer art are aware that transformer windings fail when subjected to substantial overvoltage or transient voltages. Under such currents heavy stress conditions concentrate at certain edges of the windings or coils and result in typical breakdown paths.
Where so-called "pancake" coils are involved, a single layer consists of several indentical conductors which are individually insulated in primary paper or tape and the layer of several conductors is then wrapped in secondary paper or tape. The number of secondary papers is dictated by the required impulse turn-to-turn strength. Laboratory tests of turn-to-turn impulse samples indicate that failure normally occurs at the outer ends of the layers between layers and involves only the outside conductor or strand of the layer. Pressboard insulation between the layers increases the withstand voltage, but does not affect the failure mode.
In many transformer environments available space is a factor in determining transformer dimensions, which among other things is dependent upon the coil turn height. Essentially a transformer coil comprises metal conductors and insulation. In present practice a single layer consists of several identical conductors, individually insulated, wrapped overall with the several secondary papers. To reduce the overall height of a coil turn ultimately means reducing either conductor, insulation, or both while maintaining dielectric strength.